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<center><h3>Chemical Components</h3></center>

<p><b>Chemical components</b> are the minimum number of constituents needed to
describe the composition of a chemical system. For a system with
<nobr><i>N</i><sub>spe</sub></nobr> species and <nobr><i>N</i><sub>reac</sub></nobr>
<b>independent</b> reactions, the number of components is:</p>

<center>
<nobr><i>N</i><sub>comp</sub></nobr> = <nobr><i>N</i><sub>spe</sub></nobr>
        &#8722; <nobr><i>N</i><sub>reac</sub>.</nobr>
</center>

<p>For aqueous solutions, <nobr>H<sub>2</sub>O(l)</nobr> is in excess,
and it must be selected as a component. But, because its concentration
(and activity) is approximately constant, water does not need to be included in
the component list for the calculations.</p>

<p>Any species with fixed activity should be chosen as a component. For example:
<nobr>CO<sub>2</sub>(g)</nobr> at fixed partial pressure;
<nobr>H<sup>+</sup></nobr> if the pH is used as an axis variable;
a solid phase present in large excess, etc.</p>

<p>Finally, species not capable of further dissociation are commonly selected
as components:  <nobr>Ca<sup>2+</sup>,</nobr> <nobr>CO<sub>3</sub><sup>2&#8722;</sup>,</nobr>
<nobr>PO<sub>4</sub><sup>3&#8722;</sup>,</nobr> etc. 
The total concentration for these components is usually available.</p>

<p>Make sure that
<ul><li><i>only</i> chemical components and</li>
<li><i>one</i> product</li></ul>
are involved in each reaction!</p>

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